(1) Field of the Invention
This invention relates generally to monopropellant fuel systems and to torpedo engines and pumping facilities using a monopropellant fuel. More particularly, the present invention relates to detonation traps and the use of detonation traps to prevent the combustion of the monopropellant fuel outside of the prescribed combustion chamber.
(2) Description of the Prior Art
It is known to propel a torpedo with a propulsion system which uses an external combustion expander-type engine in conjunction with a monopropellant fuel. In this type of system, a solid initiator monopropellant fuel is combusted in the combustion chamber, producing a hot, energized gas which commences drive action of the torpedo and initiates the entry of a liquid, pressure-sensitive, sustainer monopropellant fuel into the combustion chamber through a poppet valve. Assuming that the pressure in the combustion chamber is sufficiently high, heat generated in the combustion of the initiator propellant effects combustion of the initial quantity of sustainer propellant which is admitted to the combustion chamber. Subsequently, combustion of the sustainer fuel continues in a self-sustaining manner due to the high temperature and pressure in the chamber, i.e., part of the energy generated in the combustion of the sustainer monopropellant is used to combust additional sustainer monopropellant.
By the nature of their formulation, monopropellants are inherently unstable in so much as they do not require an external oxygen source to burn. The sensitivity of the monopropellant fuel increases with increasing pressure and becomes susceptible to spontaneous combustion. Extreme precautions are taken to prevent the combustion of the fuel outside of the prescribed combustion chamber. It has been observed and is well known in the art that, for a given monopropellant fuel at a given pressure, there exists a critical diameter beyond which combustion does not occur. The critical diameter decreases as the pressure increases. This data is used in developing the piping for the fuel systems used in engines utilizing monopropellant fuels. There exists the possibility that an explosion of the monopropellant fuel in the vicinity of the fuel pump may propagate back to the fuel storage tank. Periodic occurrences of fuel pump detonation have caused significant damage to pumping systems and associated engine components through such explosions. The critical diameter is used in designing detonation traps to prevent this propagation. By containing the explosion in the vicinity of the fuel pump, the detonation trap minimizes the available volume of monopropellant fuel which can participate in the explosion. Current detonation traps are designed using a series of parallel flow paths much smaller than the critical diameter determined for the specific monopropellant fuel being used.
There exists a major flaw in the design of current detonation traps in that monopropellant combustion occurs in two forms, high velocity detonation (HVD) and low velocity detonation (LVD). In the case of HVD, the present design combustion traps adequately reflect the pressure wave of the explosion and prevent the fuel tank monopropellant from entering the explosion reaction. In the case of LVD, the low propagation rate results in a pressure wave significantly less than the HVD pressure wave such that the critical diameter for a HVD does not apply. The LVD propagates through the HVD trap and detonates the monopropellant in the fuel tank. On one occasion, a LVD was determined to be the cause for the detonation of the associated fuel storage tank. When the storage fuel tank detonated, as can be expected, significant facility damage occurred. The standard HVD detonation trap failed to provide the pressure wave reflection required to isolate the fuel storage system from the LVD shock. The LVD proceeded through the HVD trap and precipitated initiation of the monopropellant in the fuel storage tank.